1. Field of the Invention
The present invention relates to a barrier mechanism which opens and closes an aperture located in front of a photographing lens in a camera.
2. Description of the Related Art
In place of a conventional removable lens cap which protects a photographing lens of a camera, a lens cap (barrier) which is provided in a camera body is used particularly in a lens shutter type of camera. There are various known barrier mechanisms, in which barriers are located in front of the photographing lens, independently of the same in function. The barriers are biased into an open position or closed position by springs or the like. A photographer operates the barrier to open or close the same against the spring force. However, such a manual operation is not desirable particularly in an automatic lens shutter type of camera in which almost all of the operations are automated.
Under these circumstances, the assignee of the present application has proposed a barrier mechanism of a camera in which a cylindrical lens cover having at its front end an aperture is linearly movably supported between a photographing position and an accommodating position located in rear of the photographing position. This is disclosed in Japanese Unexamined Utility Model Publication Nos. 62-193228 and 62-193229 (U.S. Ser. No. 144,030). The lens cover is provided, on its outer periphery, with a cam ring which moves the lens cover between the photographing position and the accommodating position in accordance with the rotation thereof. The lens cover has at its front end a pair of barriers for opening and closing the aperture and a barrier opening and closing mechanism which opens and closes the barriers in association with the rotation of the cam ring.
In these above-mentioned barrier mechanisms an opening and closing pin which is biased into a barrier opening position is displaced by a projection provided on the cam ring toward a barrier closing position. It has been found that the opening and closing pin can not be displaced by a predetermined stroke sufficient to close the barriers due to a manufacturing error or an assembly error, of the elements, thus resulting in a failure of a complete closure of the barriers.
Furthermore, in these barrier mechanisms, since the barriers, which are biased to be opened by spring means, are closed against the spring means, the largest reaction of the spring means is produced at the closed position of the barriers. Accordingly, when the cam ring is in its lens accommodating position, it is necessary to apply a large operational force to the barrier opening and closing member in order to bias the barriers into the closed position. If the operational force is decreased for some reason, the barriers can not be completely closed. In particular, the barriers are biased in a closed position by a spring means. However, when the barriers are closed the reaction of the spring means which tends to open the barriers is largest whereas the spring force which biases the closed position is at its smallest value. Thus, it is difficult to set a proper spring balance, and an impractical large spring may be needed.
In addition, there is a possibility that when the barriers are closed, they are not equally displaced, so that one of the barriers is maintained to be slightly opened. This is because, according to the inventors' analysis, the force necessary to close the barriers is not equal for each of the barriers, whereas the opening and closing member tends to move to equally close each of the barriers. Therefore, one of the barriers (heavier barrier) reaches and passes a correct closed position prior to the remaining barrier (lighter barrier), thus resulting in an excess displacement of the heavier barrier. Even if the lighter barrier comes into contact with the heavier barrier, it is impossible to bring the heavier barrier back to the correct closed position, resulting in an incomplete closure of the barriers.